The present invention relates generally to devices for modulating optical signals in a telecommunications system. More particularly, the invention concerns electro-optic modulators that are made of lithium niobate (LiNbO3) and that have an increased bandwidth and lower drive voltage than conventional modulators.
Modern telecommunications have increasingly adopted fiber optics as the medium for transmitting signals. As with electrical telecommunication signals, optical telecommunication signals can carry larger amounts of information when subjected to intensity modulation. Because much of existing electronics technology uses electrical signals, it is desirable to encode information from an electrical signal onto an optical carrier signal used in a telecommunication system.
Two approaches are commonly used to perform such electro-optic modulation: direct modulation and external modulation. Direct modulation involves varying the optical signal by directly modulating the laser diode that originates the optical signal. External modulation involves varying the optical signal after it has left the laser diode through the use of an electro-optic modulator. Unlike direct modulation, external modulation can be provided with negligible chirp, which refers to a change in carrier frequency over time.
Electro-optic external modulators work by causing the index of refraction of an optical waveguide to change in response to an applied electromagnetic signal. As the optical waveguide undergoes a variation in time of its refractive index, the optical signal passing through it is phase modulated with the corresponding electromagnetic signal. For an external modulator to achieve such a performance, the optical waveguide is formed in a material that has a strong electro-optic effect, i.e., where its optical index of refraction is easily affected by an electromagnetic signal. Typically, that material is a crystal substrate such as lithium niobate (LiNbO3). The optical waveguide is formed in the lithium niobate substrate by photolithography and diffusion of titanium. The path of titanium will have a higher index of refraction than the surrounding substrate and will constrain an optical signal within the path.
The electromagnetic signal is imparted on the external modulator through electrodes formed on the surface of the lithium niobate substrate. The electrodes are usually made of gold or a similarly conductive material and are positioned in parallel to the optical path. A portion of the electromagnetic signal travels from a xe2x80x9chotxe2x80x9d electrode to one or more ground electrodes by passing through the optical path within the lithium niobate substrate, which causes modulation of the optical signal within the path.
The positioning of electrodes with respect to optical path(s) within the modulator differs depending on whether the lithium niobate is an x-cut or a z-cut crystal. The z-axis of the lithium niobate crystal has the highest electro-optic coefficient. Consequently, the electrodes and the optical path(s) are positioned in the modulator such that the electromagnetic field passes through the optical path along the z-axis. Generally, in an x-cut substrate, the optical path(s) are positioned between the hot and ground electrodes. In a z-cut substrate, the optical path(s) are positioned directly under the electrodes.
FIGS. 1A and 1B illustrate a cross-sectional view and a top view, respectively, of a conventional external modulator in the form of a Mach-Zehnder interferometer made from a substrate of x-cut lithium niobate. As shown in FIG. 1A, lithium-niobate substrate 102 forms the base of the modulator 100 and includes two optical paths 104a and 104b. An RF signal applied between hot conductor 106 and ground electrodes 108a and 108b will cause some of the electromagnetic field to pass through optical paths 104a and 104b, modulating the optical signals passing through those paths. Optical paths 104a and 104b are positioned in this x-cut modulator so that the electromagnetic field passes through them horizontally, i.e., along the z-axis.
FIG. 1B shows the Mach-Zehnder format of the external modulator 100. The incoming optical signal travels along optical path 122 beginning at one end of the modulator and then splits at junction 124 between paths 104a and 104b. Along paths 104a and 104b, the optical signal is phase modulated as it is subjected to the RF electromagnetic field passing between electrodes 106 and 108a and 106 and 108b. At junction 126, the optical signals traveling on paths 104a and 104b are combined, and they exit modulator 100 via path 128. X-cut modulators of this type have proven effective for digital modulation at 10 Gbits/sec.
FIGS. 2A and 2B illustrate a cross-sectional view and a top view, respectively, of a conventional external modulator in the form of a Mach-Zehnder interferometer made from a substrate of z-cut lithium niobate. The same elements and references from FIG. IA apply to FIG. 2A. As shown in FIG. 2A, optical paths 104a and 104b are positioned directly beneath hot electrode 106 and ground electrode 108b. The optical paths are located in this z-cut modulator so that the electromagnetic field passes through them along substantially vertical lines, i.e., in parallel to the z-axis. FIG. 2B shows a similar arrangement for the Mach-Zehnder interferometer as in FIG. 1B except that the optical paths 104a and 104b are positioned under hot electrode 106 and ground electrode 108b. 
The z-cut crystal results in a more concentrated flux of the RF field passing through the optical paths than in an x-cut crystal. However, the improved performance of the z-cut device are mitigated by intrinsic pyroelectric problems and by a chirp parameter of approximately xe2x88x920.7, which is due to the difference in overlap between the two z-cut optical waveguides. See Wooten et al., xe2x80x9cA Review of Lithium Niobate Modulators for Fiber-Optic Communications Systems,xe2x80x9d IEEE Journal of Selected Topics in Quantum Electronics, vol. 6, no. 1, pp. 69-82 (January/February 2000).
Several performance criteria determine the quality of an external optical modulator. For example, an effective modulator must have a broad modulation bandwidth. The standard layout of an optical modulator, however, limits the modulation bandwidth because the electromagnetic signal travels more slowly through the electrodes than the optical signal travels through the optical path. This velocity mismatch introduces a phase error that is a function of the frequency of the electromagnetic signal and the length L of the interaction between the electromagnetic signal and the optical signal within the modulator. A common figure of merit for an optical modulator is the product of its xe2x88x923 dBe modulation bandwidth, which is denoted f3dB, and its interaction length L. This figure of merit should be as high as possible.
To improve velocity matching between the RF signal and the optical signal in the modulator, conventional devices include a buffer layer 120 on the surface of the lithium niobate substrate. Generally comprising SiO2 or BenzoCycloButene (BCB), buffer layer 120 lowers the dielectric constant of the material through which the RF electromagnetic field must pass, thereby increasing the velocity of the field.
Another performance concern for optical modulators is the voltage level required for the electromagnetic signal. The switching voltage necessary for a given amount of modulation should be as low as possible. The necessary voltage level is dictated in large measure by the gap G between the hot and ground electrodes and the electrode length, a small gap G decreasing the required switching voltage. A quality of modulation efficiency is expressed through the half-wave voltage Vxcfx80. For a typical Mach-Zehnder interferometer made from lithium niobate, the half-wave voltage is given by the following:                               V          π                =                              λ            ⁢                          xe2x80x83                        ⁢            G                                              n              0              3                        ⁢                          r              33                        ⁢            Γ            ⁢                          xe2x80x83                        ⁢            L                                              (        1        )            
where xcex is the free-space optical wavelength, no is the effective refractive index of the optical signal (n0≅2.14), r33 is the electro-optic coefficient, L is the electro-optic interaction length, and xcex93 is the overlap integral indicative of the degree of overlap between the electric and optical fields. See I. P. Kaminov, et al., Optical Fiber Telecommunications IIIB3, p. 396 (1997). Another common figure of merit for such an optical modulator is the product of the half-wave voltage Vxcfx80 and the interaction length L, which should be as low as possible.
A third performance criteria for optical modulators is the impedance matching between the output of the source of the modulation signal and the input of the modulator. Without impedance matching, power loss and unwanted electric signal reflections can occur. An RF signal source will typically have an output impedance of 50 ohms. To match that level, the optical modulator should be designed to increase its input impedance to a comparable value. Although ideally the input impedance should also be 50 ohms, an input impedance for the modulator of higher than 30 ohms should suffice to reduce electrical reflections below about xe2x88x9210 dB.
Increasing the input impedance of an optical modulator requires a balance between the size of the electrodes and the gap G with other performance issues. For example, widening the gap G creates a higher impedance but, as discussed above, causes the modulator to require a higher switching voltage.
Patents and publications have proposed various modifications to the standard optical modulator in an attempt to increase modulation bandwidth, decrease switching voltage, and provide impedance matching, i.e., to increase the figure of merit f3dBxc2x7L and to decrease the figure of merit Vxcfx80xc2x7L. U.S. Pat. No. 5,138,480, for example, discloses increasing the thickness xcfx84 of buffer layer 120, increasing the height t of the electrodes, and reducing the width Wh of the hot electrode and Wg of the ground electrodes such that the ratio of Wg to Wh is less than or about equal to 3.
U.S. Pat. No. 5,748,358 discloses an optical modulator that adds an electric field adjusting region between the buffer layer and the electrodes. FIG. 3, which corresponds substantially to FIG. 11 of the ""358 patent, shows a cross-section of such an x-cut modulator 300. According to the ""358 patent, electric field adjusting region 310 helps to increase the interaction between the electric field and the optical signal traveling through modulator 300 so that the driving voltage can be lowered.
Applicants have found, however, that the structure proposed in the ""358 patent suffers from manufacturing deficiencies. In particular, the ""358 patent discloses that to have electrical-optical velocity matching, the width Wh of hot electrode 106 should be smaller than the width Wf of the optical path. According to the ""358 patent, when the hot electrode is designed to have a width Wh of 10-20 xcexcm, the effective refractive index for the electric field becomes unacceptably larger than the refractive index for the optical signal. In the preferred embodiment of the ""358 patent, the width Wh is set to 5 xcexcm.
Koshiba, xe2x80x9cFinite-Element Modeling of Broad-Band Traveling-Wave Optical Modulators,xe2x80x9d IEEE Transactions on Microwave Theory and Techniques, vol. 47, no. 9, pp. 1627-33 (September 1999) describes a full-wave finite element modeling method with hybrid edge/nodal elements. When this finite element modeling is applied to the modulator of the ""358 patent, it can be proved that for the modulator to have a width Wh of 5 xcexcm and for electrical-optical velocity matching to occur, thickness t of the electrodes must be at least 25 xcexcm. The electrodes are commonly formed by an electroplating method. Therefore, to achieve a metal thickness of 25 xcexcm, a photoresist thicker than 25 xcexcm would first need to be applied on the substrate surface, and then portions of it corresponding to the electrodes would need to be selectively exposed and removed. Due to the great difference between the width of the hot electrode Wh and the thickness t, the light exposure undercuts the photoresist and results in an opening with a trapezoidal shape, as shown in FIG. 4. As a result, manufacture of the modulator proposed in the ""358 patent results in poor reproducibility (the growth angle being variable, for example between about 4 and 5 degrees) of the electroplated electrode. Therefore, the impedance and the effective refractive index for the electromagnetic signal will vary uncontrollably from chip to chip. Moreover, the large thickness of the metal drastically reduces the characteristic impedance of the modulator.
To date, broadband modulators that have low driving voltages have been experimentally achieved without increasing the buffer layer or electrode thickness, but they require formation of recesses within the lithium niobate substrate. For example, Kaminow et al., xe2x80x9cLithium Niobate Ridge Waveguide Modulator,xe2x80x9d Applied Physics Letters, vol. 24, no. 12, pp. 622-24 (Jun. 15, 1974) generally describes a device that uses a narrow ridge etched into the lithium niobate. Unfortunately, the manufacturing process for these devices becomes very complicated, and the formation of recesses may damage the substrate.
Similarly, Madabhushi et al., xe2x80x9cWide-Band, Low Driving Voltage Ti:LiNbO3 Optical Modulators for 40 Gb/s Applications,xe2x80x9d ECOC ""98, pp. 547-48 (Sep. 20-24, 1998) describes an optical modulator with a so-called step-buffer layer structure. In this article, the authors propose a modulator structure that has an SiO2 buffer layer that is partially etched so that it has one thickness over one half of the waveguide and another thickness over the other half. See also G. K. Gopalakrishnan, et al., xe2x80x9c40 GHz, Low Half-Wave Voltage Ti:LiNbO3 Intensity Modulator,xe2x80x9d Electronics Letters, Vol. 28, pp. 826-27 (April 1992).
Other proposals to modify conventional modulator structures to improve optical-electrical velocity matching, driving voltage, and impedance matching have faced conflicting outcomes. For example, an increase of the buffer layer thickness xcfx84 has the positive effects of lowering the refractive index for the electromagnetic field (which is typically higher than the refractive index for the optical signal) and increasing the impedance as well as the bandwidth, but it also results in an increase of the half-wave voltage. If the electrode thickness t is increased, optical-to-electrical velocity mismatch is desirably reduced, but characteristic impedance is undesirably reduced as well. Similarly, an increase of the electrode gap G decreases the electric field strength, which increases the product Vxcfx80xc2x7L, increases the impedance, and decreases electromagnetic attenuation. An increased gap G, however, diminishes the positive effect of the thick buffer layer, increasing the refractive index for the electromagnetic field.
Applicants have observed that conventional optical modulators do not provide adequate adjustment of the refractive index or dielectric constant for the electromagnetic signal and increase in the characteristic impedance while also permitting easy and consistent manufacturability. Applicants have further observed that the existing proposals for single-layer ground electrodes in an optical modulator do not provide sufficiently high bandwidth and low drive voltage for future applications.
Applicants have found that the drawbacks of conventional optical modulators can be overcome with an optical modulator that incorporates more than one layer for the hot electrode and the ground electrodes. Applicants has in particular discovered that an optical modulator having an upper layer for ground electrodes that is narrower in width than a lower layer can provide desirable electrical-optical velocity matching and desirable electrical impedance matching.
In a first aspect, an optical modulator consistent with the principles of the present invention that imparts intensity modulation on an optical carrier signal in response to an electromagnetic drive signal includes a substrate of a material having an electro-optic effect, an optical path formed within the substrate, and preferably a buffer layer formed on top of the substrate. The buffer layer is a material having a dielectric constant lower than the substrate.
On top of the substrate or of the buffer layer, the optical modulator further includes a hot electrode and at least one ground electrode, where both electrodes extend substantially parallel to at least a portion of the optical path. The hot electrode includes a lower hot electrode positioned above the buffer layer and an upper hot electrode positioned on top of the lower electrode, where the upper hot electrode is preferably smaller in width than the lower hot electrode. The ground electrode includes a lower ground electrode positioned above the buffer layer and separated from the lower hot electrode by a gap G1, and an upper ground electrode positioned on top of the lower ground electrode and separated from the upper hot electrode by a gap G2 larger than G1. The upper ground electrode is smaller in width than the lower ground electrode.
In a second aspect, a Mach-Zehnder interferometer consistent with the principles of the present invention can be formed on a substrate of lithium niobate and includes an optical path of diffused titanium within the substrate, a hot electrode, and a ground electrode. The hot electrode is positioned on top of the substrate and is made of a first layer overlapped by a second layer, where the first layer is larger in width and smaller in height than the second layer. The ground electrode is also positioned on top of the substrate and is made of a first layer overlapped by a second layer, where the first layer being larger in width and smaller in height than the second layer. Both the hot electrode and the ground electrode extend substantially parallel to each other and to the optical path along an interaction length.
In a thirds aspect, a method of making a Mach-Zehnder interferometer and, in particular, an optical modulator, consistent with the principles of the present invention, includes diffusing titanium into a substrate of lithium niobate in a confined region to form an optical path through the substrate, preferably depositing a buffer layer on a top surface of the substrate of a material having a dielectric constant lower than a dielectric constant of the substrate, electroplating a first pattern of electrically conductive material on the surface, and electroplating a second pattern of electrically conductive material on top of the first pattern. The first pattern includes a hot electrode and at least one ground electrode. The second pattern is narrower than the first pattern over the hot electrode and the at least one ground electrode, respectively. Both electrodes are parallel to the optical path along an interaction length.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of the invention.